As an electronic medium which is an alternative to paper, an electronic paper is actively developed. Contrary to the conventional displays, such as a CRT and a liquid crystal display, necessary characteristics of the electronic paper include being a reflection type display element, having a high white reflectance and a high contrast ratio, and, having a memory effect for display. The characteristics also include being capable of being driven under a low voltage, being thin and light, and being low in price. Specifically, as display characteristics, a white reflectance and a contrast ratio equivalent to those of paper are required. There is a great demand for colorization of the electronic paper because, as a matter of course, the conventional paper media display full color.
Among the technologies proposed so far, as electronic paper capable of performing color display, there has already been commercialized a medium in which a color filter is formed, for example, on a reflection type display element. However, this product uses a polarizing plate and therefore utilization efficiency of light is low, and only dark white color can be displayed. Further, because black color cannot be displayed, a contrast ratio is poor.
Moreover, there is an electrophoresis system based on a principle in which charged white particles as a bright reflection type display element and black particles are moved by electric fields. However, it is said that the scattering reflectance of the white particles in this system is a little less than 40% at most, and it is required to further improve the reflection efficiency. Further, when colorization is performed, the reflection efficiency is degraded, and hence bright color electronic paper is greatly desired.
For example, Patent Literature 1, Patent Literature 2, Patent Literature 3, and Patent Literature 4 disclose a reflection type color display medium in which color filters with electrophoretic elements are formed. If color filters having black matrixes, which are used in the conventional liquid crystal displays, or colored pixels are superposed on these reflection type color display medium, their brightness is deteriorated. Moreover, in Patent Literature 1, when a multicolor display element is realized, the colored pixels are formed by the same number of steps of photolithography system as the number of the colored layers, and hence the cost of steps and the colored resist are wastefully used.
On the other hand, in the method of producing a color filter by an inkjet method, red, blue, and green inks are ejected simultaneously only at respective necessary pixels and applied to regions where pixels are formed, and cured so as to form pixels. There has been proposed a method involving preliminarily forming partition walls by a photolithography step, and ejecting ink to a pixel portion (Patent Literature 5 and Patent Literature 6). In order to avoid blurring of each color region and mixture of colors between adjacent regions in this method, for example, Patent Literature 7 describes an example in which mixture of colors can be avoided when a static contact angle between the ink and the partition wall surface is 30 to 65°. This method decreases the cost of steps and the colored resist in comparison to the photolithography method described above.
Further, there has been proposed means for forming colored layers without forming a partition wall when producing a color filter by an inkjet method (Patent Literature 8). However, in this means, a black matrix layer is preliminarily formed as an undercoating, and a selective application of ink is performed by this region. Further, electronic paper is a reflection type display element and a black matrix is not needed.